Hip resurfacing is commonly performed using acetabular cups and femoral components which are made from solid metal. However, it has been estimated that approximately 1% of patients who undergo such metal-on-metal hip resurfacing have a pseudo-tumour in the form of a soft tissue mass or large symptomatic effusion within 5 years. The symptoms of these pseudo-tumours include discomfort, spontaneous dislocation, nerve palsy, a noticeable mass and a rash, while the common histological features are extensive necrosis and lymphocytic infiltration. As a consequence, many patients require revision surgery followed by conventional total hip replacement.
Whilst the cause of these pseudo-tumours is currently unconfirmed, it has been observed that they occur in situations of high bearing wear. This could be caused by poor wearing metal as a result of non-optimal heat treatment during processing or due to component misalignment, which may either result from the surgeon mal-positioning one or more of the components or from an underlying bony misalignment of the skeleton (e.g. developmental dysplasia of the hip). Edge wear of the acetabular component has also been observed along with excessive wear of the femoral component due to impingement.
It is believed that the pseudo-tumours may, in fact, be due to a toxic reaction to an excess of particulate metal wear debris or metal ions or, perhaps, a hypersensitivity reaction to a normal amount of metal wear debris. There is therefore a concern that, with time, the incidence of these pseudo-tumours may increase.
Other materials have been considered for use in hip resurfacing. For example, a metal outer cup shell has been combined with a polymer (e.g. conventional non cross-linked polyethylene) inner cup liner. However, in these instances an even higher failure rate is encountered because wear of the bearing surface leads to early loosening of the joint and the production of large quantities of polymer debris. This results in osteolysis of the acetabulum and femur making revision surgery difficult due to the loss of bone stock.
Commonly, acetabular cups are configured for press-fit fixation (e.g. by forcing a 50 mm outer diameter component into a 48 mm diameter hole). This can result in considerable deformation (e.g. in the range of 100 microns to over 350 microns), even where thick metal shells are employed, and so there is a risk that the cup will grip the femoral component leading to early acetabular component breakout. Alternative fixation features, such as large projecting pegs, are therefore sometimes employed. To aid fixation of these components, the external surface of the cup is often provided with a porous coating (e.g. by plasma spraying titanium particles) to encourage bone in-growth. However, such a coating only tends to provide limited contact between the bone and the metal coating leading to poor grip. In addition, the titanium particles can easily become dislodged, such that they then serve as abrasive debris.
It is therefore an aim of the present invention to provide a method of forming a polymer component (e.g. for a prosthesis such as an acetabular cup prosthesis), which helps to ameliorate some of all of the afore-mentioned problems.